1. Field of the Invention
The present invention relates to a mobile communication system, and more particularly to a symbol synchronization method for an orthogonal frequency division multiplexing (OFDM)-based wireless communication system.
2. Description of the Related Art
OFDM schemes are suitable for high speed data transmission through a wire/wireless channel. Recently, research into the OFDM scheme has been actively pursued.
FIG. 1 is a diagram illustrating a general data transmission/reception process in an OFDM system. As shown in FIG. 1, in an OFDM scheme, input data is serial-to-parallel converted based on the number of subcarriers, the output parallel data is modulated through an inverse fast fourier transform (IFFT), the modulated data is multiplexed on a time domain, a guard interval (GI) is inserted into the multiplexed data, and the multiplexed data is then transmitted. A reception-side restores the transmitted data through a fast fourier transform (FFT). Generally, in order to prevent inter carrier interference (ICI) from occurring in a predetermined FFT window due to a synchronization error, a cyclic prefix (CP) obtained by copying a last portion of an effective symbol interval is inserted and used as a guard interval.
In an OFDM scheme, a symbol period increases as the number of subcarriers increases, while data transmission speed is maintained through an IFFT. Further, since an OFDM scheme uses a subcarrier having a mutual orthogonality, the OFDM scheme has a bandwidth efficiency that is high as compared with the conventional frequency division multiplexing (FDM). Furthermore, since the OFDM scheme has a long symbol period, the OFDM scheme is strong against interference between symbols as compared with a single carrier modulation scheme.
Generally, a modulation/demodulation of an OFDM signal is efficiently performed through an IFFT/FFT or an inverse discrete cosine transform (IDCT)/discrete cosine transform (DCT). However, since data modulated by means of an IFFT in a modulation process may be restored into original data through an FFT of a reception-side, it is required to provide physical layer modules corresponding to the number of radio access routers (RARs) which transmit data in order to simultaneously receive the data from different RARs. In other words, when a terminal must maintain connections with two RARs in a particular situation such as a soft handover, the terminal must perform an FFT for each connection in order to maintain the connections with two RARs.
Since an OFDM-based data transmission/reception scheme modulate/demodulate data by means of an IFFT/FFT, a terminal must separately process frames received from a plurality of radio access routers (RARs) in order to receive data from different RARs.
A multi-connection method by which a terminal including two physical layer modules processes signals received from two RARs is disclosed in patent publication a WO03017689 to Laroia, et al.
FIG. 2 is a block diagram illustrating a multi-connection method for supporting the mobility of a terminal in the conventional OFDM-based system and FIG. 3 is a block diagram showing the construction of a terminal for a multi-connection in the system of FIG. 2.
In FIG. 2, the terminal 302 maintains connections 410 and 414 with two RARs 304 and 306. The connections 410 and 414 include upward control links 408 and 412 and downward control links 409 and 413, and upward data links 416 and 418 and downward data links 417 and 419.
In order to connect with two RARs as described above, the terminal 900 includes an analog processing module 902, an analog/digital converter 904, a copy module 906, a pair of signal separating circuits 905 and 907, a pair of synchronization loops 908 and 909, and a pair of main digital processing modules 912 and 914, as shown in FIG. 3.